Aldehydes are the customary starting materials for obtaining carboxylic acids. The position of preference for this field of use is thanks to their availability in great variety and the ease of conversion of the carbonyl group into the carboxyl group by oxidation. In processes carried out industrially, the conversion of aldehydes into carboxylic acids is carried out either in the presence or in the absence of catalysts or additives. Possible catalysts are predominantly salts of transition metals, in particular salts of cobalt and of manganese and also of chromium, iron, copper, nickel, silver and vanadium. The formation of carboxylic acid from aldehydes is frequently associated with secondary reactions and degradation reactions even when optimal temperature conditions are adhered to. This applies equally for reactions in the presence and absence of catalysts. In such cases, the selectivity of the reaction can be improved considerably by use of alkali metal salts or alkaline earth metal salts of weak acids as additives (Ullmanns Encyclopadie der technischen Chemie, 4th edition 1975 volume 9, page 139).
Particularly in the oxidation of aliphatic, α-alkyl-branched aldehydes in which the carbon atom adjacent to the carbonyl carbon bears the alkyl branch, the prior art recommends the addition of small amounts of alkali metal carboxylates to improve the selectivity. Thus, for example, it is known from DE 950 007 that the oxidation of aldehydes branched in the a position requires the addition of small amounts of alkali metal salts of carboxylic acids in order to obtain the desired carboxylic acid in high yield and also high purity. It is known from U.S. Pat. No. 5,504,229 that the alkali metal-containing distillation residue obtained in the distillation of α-alkyl-branched carboxylic acids can be reused for the aldehyde oxidation. It is also stated that the α-alkyl-branched carboxylic acid can be liberated from the distillation residue by acidification. However, the subsequently purified carboxylic acid displays only a moderate colour number.
According to the teaching of the published Japanese patent application 53-105413, aliphatic, α-branched aldehydes are oxidized by means of oxygen in the presence of lithium compounds or alkaline earth metal compounds which are used in amounts of from 0.01 to 10% by weight, based on the total reaction system, in order to prepare aliphatic, α-branched carboxylic acids. The low-temperature oxidation process described in the French patent application 2 769 624 is also carried out in the presence of alkali metal compounds or alkaline earth metal compounds as additives. DE-C1-100 10 771 discloses both the sole use of alkali metal salts and the use of these in admixture with transition metals in the oxidation of 2-methylbutanal.
In the oxidation of aliphatic straight-chain or branched aldehydes which do not bear an alkyl branch in the α position, too, the use of a mixture of alkali metal carboxylates or alkaline earth metal carboxylates with transition metals has been described. DE 10 2004 055 252 A1 discloses the oxidation of n-pentanal or of isononanal based on 3,5,5-trimethylhexanal in the presence of the corresponding potassium carboxylate and iron. The crude acid obtained after oxidation is separated off by distillation and the metal-containing distillation residue obtained can be reused in the aldehyde oxidation. According to the teaching of DE 10 2006 022 168 A1, a mixture of alkali metal carboxylates or alkaline earth metal carboxylates and transition metals is prepared as reaction product in a first aldehyde oxidation reaction and this reaction product is reused for the subsequent oxidation of aliphatic straight-chain or β-alkyl-branched monocarboxylic acids.
It is usual for the respective carboxylic acid firstly to be reacted in a separate reaction with an aqueous solution of an alkali metal compound or alkaline earth metal compound, preferably with an aqueous alkali metal hydroxide or alkaline earth metal hydroxide solution, to convert it into the respective carboxylate which is mixed into the aldehyde to be oxidized. An aqueous alkali metal hydroxide or alkaline earth metal hydroxide solution can also be added to the reaction mixture so that the formation of the respective carboxylates occurs during the oxidation. The use of potassium carboxylates has been found to be particularly useful. After the oxidation reaction is complete, the crude acid is worked up by distillation, giving a highly viscous distillation residue containing alkali metal carboxylate or alkaline earth metal carboxylate. This distillation residue can to a certain extent be recirculated to the oxidation process.
However, as the amount recirculated increases, the selectivity of the aldehyde oxidation decreases and the distillation residue ultimately has to be discharged from the process.